The invention relates to a synchronous motor comprising a stator and a rotor rotor; and more particularly to a motor whose rotor comprises two coaxial rotor sections which are each provided with rotor teeth at the outer circumference, the teeth being arranged along a circle and being axially spaced by an interposed axially magnetized permanent magnet. The stator comprises two coaxial stator sections which are interconnected in a magnetically conductive fashion and which are each provided with at least one coaxial annular coil which is enclosed by a magnetically conductive circuit which comprises a rotor section and the stator section which surrounds the relevant annular coil. Each stator section terminates in two sets of coaxially arranged stator teeth which cooperate with the rotor teeth via air gaps, one of said sets of stator teeth being oriented towards the cooperating rotor teeth and the other sets of stator teeth being offset from the cooperating rotor teeth by substantially 1/4, 1/2 and 3/4 tooth pitch in such a way that a difference in offset of 1/2 tooth pitch is obtained between the stator sections.
Such a synchronous motor, which may be constructed as a stepping motor, is of the hybrid type and is described, for example, in U.S. Pat. No. 4,206,374. In the motor described therein the magnet is situated in the stator, but it is stated that this magnet may alternately be arranged in the rotor. The various tooth offsets in said motor are obtained by arranging the stator teeth at positions which are situated 0, 1/2, 1/4 and 3/4 tooth pitch from each other and arranging the rotor teeth in line. In a version of the motor which is marketed by N. V. Philips' Gloeilampenfabrieken and which is described in Philips Data Handbook Components and Materials, Book C17, Stepping motors and associated electronics, 1984, pages 8 and 9, the offset is obtained by arranging the rotor teeth at positions which are situated 0, 1/2, 1/4 and 3/4 tooth pitch from each other and arranging the stator teeth in line.
Such a stepping motor has the disadvantage that in the stator five different parts must be interconnected, namely two parts per stator section (to allow the coils to be mounted) with a permanent magnet interposed between these stator sections, which leads to a weak construction. Moreover, the alignment of the four different tooth positions is sometimes an intricate operation. In the case of 50 teeth per revolution the stepping angle is 1.8.degree.. A stepping-angle accuracy of, for example, 3% then demands an alignment accuracy of approximately 3 minutes of arc. In the commercially available motor the latter problem has been solved by providing the tooth offset on the rotor and forming the rotor teeth in four rings on said rotor, the rotor being rotated through 1/4 tooth pitch every time. However, this requires a specific axial clearance between these four sets of teeth, which clearance becomes too small when the motor dimensions are reduced. The rotor must then be assembled from four separate parts which, apart from the weak construction, has the disadvantage that four parts must be secured to each other at four different angles with the said accuracy.